This proposal describes a 5-year program for the development of a career in academic molecular pathology. Dr. Robinson pursued post-doctoral studies on cell signaling at Harvard and pathology training at Duke;she is now a tenure-track Assistant Professor of Pathology at the University of Pittsburgh and fully committed to a research career focused on mechanistic analyses of the regulation of bone and bone marrow cells. Dr. Robinson's mentor for this proposal, a Professor of Pathology at Pittsburgh, is an authority on the regulation of bone resorption and is guiding her new endeavors in the study of osteoclast formation. The long-term goal of the applicant is to understand the interactions between the hematologic and stromal components of the bone marrow and the mechanisms of their reciprocal regulation. The proposed research examines nongenomic estrogen effects on osteoclast formation involving the protein BCAR1. Preliminary studies revealed that estrogen triggers the binding of its receptor to the scaffolding protein BCAR1. This association is sensitive to receptor agonists and modulators, as well as to RANKL, the major stimulus for osteoclast differentiation. RANKL stimulated osteoclast differentiation is inhibited by estrogen through acute effects on RANKL signal transduction. When BCAR1 expression is inhibited, however, estrogen effects on RANKL signaling are abrogated. In addition, BCAR1 interacts with RANKL signaling in the absence of estrogen, and in its absence, osteoclast formation is inhibited. These results indicate that a complex network of interactions links BCAR1 to both estrogen and RANKL. Proposed studies test the hypothesis that BCAR1 mediates nongenomic estrogen effects in pre-osteoclasts by forming a complex with the estrogen receptor that interacts with RANKL signaling mechanisms. The specific aims are to: define components of the ER-BCAR1 complex and their regulation by estrogen and RANKL, and identify the molecular mechanisms by which BCAR1, and estrogen through ER-BCAR1, modulate RANKL signaling to regulate osteoclast formation. Clinical significance: these studies will define the molecular mechanisms by which estrogen inhibits bone resorption (anti-osteoporosis effect). They focus on BCAR1, a protein implicated in estrogen effects on breast cancer. Understanding the mechanisms by which this protein functions in bone is important for the design of therapies that treat osteoporosis without exacerbating hormone dependent tumor formation or progression.